Vehicle windshields comprise a laminated structure which generally includes a pair of outer sheets of silicate glass and an inner thermoplastic, synthetic, high polymer sheet located therebetween for purposes as are well-known in the automotive arts. Customarily, the plastic material to be used in the fabrication of a windshield is supplied as a "rough" section, that is, a rectangular section which theretofore was cut from an unwound length of a rolled strip. The various lamina, then, are assembled under heat and pressure prior to further processing steps.
Present day windshields generally are of trapezoidal shape, and their parallel bases are curved in a circular manner as a function of the shape of the vehicle with which the windshield is used. Oftentimes, the windshield will include a filtering zone within the region of the upper base for purposes of filtering glare. The filtering zone preferably comprises a strip which extends laterally of the windshield along the length of the upper base and may be formed by tinting or coloring the plastic within that region.
If the filtering zone is to extend in a visually horizontal attitude when the windshield is in place on the vehicle, that is, with the boundary line between the tinted and untinted regions of the windshield parallel to the upper base of the windshield, heretofore indicated as being curved, for any reason, aesthetic or otherwise, such as for example, to comply with a visibility standard, then the plastic material of the "rough" or rectangular section must be shaped so that the boundary line generally follows the curve of the upper base. If the plastic material is not shaped the boundary line will not be parallel to the curve of the upper base.
Several shaping techniques have been resorted to within the prior art. To this end, U.S. Pat. No. 3,341,889 discloses that the plastic material of the "rough" or rectangular section, after it is cut, from an unwound length, is heated to bring it to a desired temperature, and, then, the section of plastic material is deformed by exerting unequal forces on the same.
German Offenlegungsschrift No. 27 42 897 discloses another technique for shaping a plastic material for use in a windshield having upper and lower curved bases. The German publication discloses that the plastic material in the form of a strip, brought to and maintained at the deformation temperature, is wound on a conical drum. The winding operation is carried out while subjecting the plastic material to stress. The conical surface of the drum is indicated as providing the plastic material with a desired curvature.
U.S. Pat. No. 3,038,208 represents a further example of the prior art. This patent discloses the transport of a continuous strip of plastic material to and through a zone of heating, heating of a region of the plastic material, and thereafter, drawing the heated plastic material to a state of permanent deformation.
The prior art, set out above, is considered to suffer from various problems and disadvantages. For example, the technique described in the first-mentioned patent is practiced on individual rectangular sections and, consequently, is considered to be uneconomical. The process of the German publication is considered to suffer from a requirement of use of specifically contoured conical drums or mandels which ultimately result in difficulties in storage and shipping of plastic material on the conical mandrel, and a lack of universality of operation. Thus, while the process may be carried out continuously, because of difference in the width dimension between the long edges of the windshield and the required curvature to be obtained, the process requires the use of conical mandrels whose end diameters must differ as a function of the desired degree of curvature. The last-mentioned patent overcomes those difficulties relating to the storage and shipping of plastic material on conical mandrels, but suffers from problems which reside in the completion of the overall process whereby the plastic material, once it is deformed, is immediately cut into sections of desired length. These deformed sections are more difficult to handle, store and ship, which difficulties add to the expense of fabrication.